A reduced number of hippocampal granule cells does not associate with an anhedonia-like phenotype in a rat chronic mild stress model of depression

'Unpredictable chronic mild stress does not exacerbate memory impairment or altered neuronal and glial plasticity in the hippocampus of middle-aged vitamin D deficient mice'

Vitamin D deficiency is a worldwide health concern, especially in the elderly population. Much remains unknown about the relationship between vitamin D deficiency (VDD), stress-induced cognitive dysfunctions and depressive-like behaviour. In this study, 4-month-old male C57Bl/6J mice were fed with control or vitamin D free diet for 6 months, followed by unpredictable chronic stress (UCMS) for 8 weeks. VDD induced cognitive impairment and reduced grooming behaviour, but did not induce depressive-like behaviour. While UCMS in vitamin D sufficient mice induced expected depressive-like phenotype and impairments in the contextual fear memory, chronic stress did not manifest as an additional risk factor for memory impairments and depressive-like behaviour in VDD mice. In fact, UCMS restored self-care behaviour in VDD mice. At the histopathological level, VDD mice exhibited cell loss in the granule cell layer, reduced survival of newly generated cells, accompanied with an increased number of apoptotic cells and alterations in glial morphology in the hippocampus; however, these effects were not exacerbated by UCMS. Interestingly, UCMS reversed VDD induced loss of microglial cells. Moreover, tyrosine hydroxylase levels decreased in the striatum of VDD mice, but not in stressed VDD mice. These findings indicate that long-term VDD in adulthood impairs cognition but does not augment behavioural response to UCMS in middle-aged mice. While VDD caused cell loss and altered glial response in the DG of the hippocampus, these effects were not exacerbated by UCMS and could contribute to mechanisms regulating altered stress response.

The role of the circadian system in the etiology of depression

Circadian rhythms have evolved in almost all organisms enabling them to anticipate alternating changes in the environment. As a consequence, the circadian clock controls a broad range of bodily functions including appetite, sleep, activity and cortisol levels. The circadian clock synchronizes itself to the external world mainly by environmental light cues and can be disturbed by a variety of factors, including shift-work, jet-lag, stress, ageing and artificial light at night. Interestingly, mood has also been shown to follow a diurnal rhythm. Moreover, circadian disruption has been associated with various mood disorders and patients suffering from depression have irregular biological rhythms in sleep, appetite, activity and cortisol levels suggesting that circadian rhythmicity is crucially involved in the etiology and pathophysiology of depression. The aim of the present review is to give an overview and discuss recent findings in both humans and rodents linking a disturbed circadian rhythm to depression. Understanding the relation between a disturbed circadian rhythm and the etiology of depression may lead to novel therapeutic and preventative strategies.

The times they are a-changin': a proposal on how brain flexibility goes beyond the obvious to include the concepts of "upward" and "downward" to neuroplasticity

Since the brain was found to be somehow flexible, plastic, researchers worldwide have been trying to comprehend its fundamentals to better understand the brain itself, make predictions, disentangle the neurobiology of brain diseases, and finally propose up-to-date treatments. Neuroplasticity is simple as a concept, but extremely complex when it comes to its mechanisms. This review aims to bring to light an aspect about neuroplasticity that is often not given enough attention as it should, the fact that the brain's ability to change would include its ability to disconnect synapses. So, neuronal shrinkage, decrease in spine density or dendritic complexity should be included within the concept of neuroplasticity as part of its mechanisms, not as an impairment of it. To that end, we extensively describe a variety of studies involving topics such as neurodevelopment, aging, stress, memory and homeostatic plasticity to highlight how the weakening and disconnection of synapses organically permeate the brain in so many ways as a good practice of its intrinsic physiology. Therefore, we propose to break down neuroplasticity into two sub-concepts, "upward neuroplasticity" for changes related to synaptic construction and "downward neuroplasticity" for changes related to synaptic deconstruction. With these sub-concepts, neuroplasticity could be better understood from a bigger landscape as a vector in which both directions could be taken for the brain to flexibly adapt to certain demands. Such a paradigm shift would allow a better understanding of the concept of neuroplasticity to avoid any data interpretation bias, once it makes clear that there is no morality with regard to the organic and physiological changes that involve dynamic biological systems as seen in the brain.

The hippocampus in stress susceptibility and resilience: Reviewing molecular and functional markers

Understanding the individual variability that comes with the likelihood of developing stress-related psychopathologies is of paramount importance when addressing mechanisms of their neurobiology. This article focuses on the hippocampus as a region that is highly influenced by chronic stress exposure and that has strong ties to the development of related disorders, such as depression and post-traumatic stress disorder. We first outline three commonly used animal models that have been used to separate animals into susceptible and resilient cohorts. Next, we review molecular and functional hippocampal markers of susceptibility and resilience. We propose that the hippocampus plays a crucial role in the differences in the processing and storage of stress-related information in animals with different stress susceptibilities. These hippocampal markers not only help us attain a more comprehensive understanding of the various facets of stress-related pathophysiology, but also could be targeted for the development of new treatments.

Dissecting the role of adult hippocampal neurogenesis towards resilience versus susceptibility to stress-related mood disorders

Adult hippocampal neurogenesis in the developmental process of generating and integrating new neurons in the hippocampus during adulthood and is a unique form of structural plasticity with enormous potential to modulate neural circuit function and behaviour. Dysregulation of this process is strongly linked to stress-related neuropsychiatric conditions such as anxiety and depression, and efforts have focused on unravelling the contribution of adult-born neurons in regulating stress response and recovery. Chronic stress has been shown to impair this process, whereas treatment with clinical antidepressants was found to enhance the production of new neurons in the hippocampus. However, the precise role of adult hippocampal neurogenesis in mediating the behavioural response to chronic stress is not clear and whether these adult-born neurons buffer or increase susceptibility to stress-induced mood-related maladaptation remains one of the controversial issues. In this review, we appraise evidence probing the causal role of adult hippocampal neurogenesis in the regulation of emotional behaviour in rodents. We find that the relationship between adult-born hippocampal neurons and stress-related mood disorders is not linear, and that simple subtraction or addition of these neurons alone is not sufficient to lead to anxiety/depression or have antidepressant-like effects. We propose that future studies examining how stress affects unique properties of adult-born neurons, such as the excitability and the pattern of connectivity during their critical period of maturation will provide a deeper understanding of the mechanisms by which these neurons contribute to functional outcomes in stress-related mood disorders.

The Impact of Chronic Unpredictable Mild Stress-Induced Depression on Spatial, Recognition and Reference Memory Tasks in Mice: Behavioral and Histological Study

Depression-induced cognitive impairment has recently been given more attention in research. However, the relationship between depression and different types of memory is still not clear. Chronic unpredictable mild stress (CUMS) is a commonly used animal model of depression in which animals are exposed to chronic unpredictable environmental and psychological stressors, which mimics daily human life stressors. This study investigated the impact of different durations of CUMS on various types of memory (short- and long-term spatial memory and recognition memory) and investigated CUMS’ impact on the ultrastructural level by histological assessment of the hippocampus and prefrontal cortex. Twenty male C57BL/J6 mice (6 weeks old, 21.8 ± 2 g) were randomly divided into two groups (n = 10): control and CUMS (8 weeks). A series of behavioral tasks were conducted twice at weeks 5–6 (early CUMS) and weeks 7–8 (late CUMS). A tail-suspension test (TST), forced swimming test (FST), elevated zero maze (EZM), elevated plus maze (EPM), open field test (OFT), and sucrose-preference test (SPT) were used to assess anxiety and depressive symptoms. The cognitive function was assessed by the novel object recognition test (NORT; for recognition memory), Y-maze (for short-term spatial memory), and Morris water maze (MWM: for long-term spatial memory) with a probe test (for reference memory). Our data showed that 8 weeks of CUMS increased the anxiety level, reported by a significant increase in anxiety index in both EPM and EZM and a significant decrease in central preference in OFT, and depression was reported by a significant increase in immobility in the TST and FST and sucrose preference in the SPT. Investigating the impact of CUMS on various types of memory, we found that reference memory is the first memory to be affected in early CUMS. In late CUMS, all types of memory were impaired, and this was consistent with the abnormal histological features of the memory-related areas in the brain (hippocampus and prefrontal cortex).

Chronic mild stress paradigm as a rat model of depression: facts, artifacts, and future perspectives

RATIONALE

The chronic mild stress (CMS) paradigm was first described almost 40 years ago and has become a widely used model in the search for antidepressant drugs for major depression disorder (MDD). It has resulted in the publication of almost 1700 studies in rats alone. Under the original CMS procedure, the expression of an anhedonic response, a key symptom of depression, was seen as an essential feature of both the model and a depressive state. The prolonged exposure of rodents to unpredictable/uncontrollable mild stressors leads to a reduction in the intake of palatable liquids, behavioral despair, locomotor inhibition, anxiety-like changes, and vegetative (somatic) abnormalities. Many of the CMS studies do not report these patterns of behaviors, and they often fail to include consistent molecular, neuroanatomical, and physiological phenotypes of CMS-exposed animals.

OBJECTIVES

To critically review the CMS studies in rats so that conceptual and methodological flaws can be avoided in future studies.

RESULTS

Analysis of the literature supports the validity of the CMS model and its impact on the field. However, further improvements could be achieved by (i) the stratification of animals into 'resilient' and 'susceptible' cohorts within the CMS animals, (ii) the use of more refined protocols in the sucrose test to mitigate physiological and physical artifacts, and (iii) the systematic evaluation of the non-specific effects of CMS and implementation of appropriate adjustments within the behavioral tests.

CONCLUSIONS

We propose methodological revisions and the use of more advanced behavioral tests to refine the rat CMS paradigm, which offers a valuable tool for developing new antidepressant medications.

Reviewing the Role of the Endocannabinoid System in the Pathophysiology of Depression

Major depressive disorder is a high-impact, debilitating disease and it is currently considered the most prevalent mental illness. It is associated with disability, as well as increased morbidity and mortality. Despite its significant repercussions in our society, its exact pathophysiology remains unclear and therefore, available antidepressant treatment options are limited and, in some cases, ineffective. In the past years, research has focused on the development of a multifactorial theory of depression. Simultaneously, evidence supporting the role of the endocannabinoid system in the neurobiology of neuropsychiatric diseases has emerged. Studies have shown that the endocannabinoid system strongly impacts neurotransmission, and the neuroendocrine and neuroimmune systems, which are known to be dysfunctional in depressive patients. Accordingly, common antidepressants were shown to have a direct impact on the expression of cannabinoid receptors throughout the brain. Therefore, the relationship between the endocannabinoid system and major depressive disorder is worth consideration. Nevertheless, most studies focus on smaller pieces of what is undoubtedly a larger mosaic of interdependent processes. Therefore, the present review summarizes the existing literature regarding the role of the endocannabinoid system in depression aiming to integrate this information into a holistic picture for a better understanding of the relationship between the two.

Insufficiency of ventral hippocampus to medial prefrontal cortex transmission explains antidepressant non-response

BACKGROUND

There is extensive evidence that antidepressant drugs restore normal brain function by repairing damage to ventral hippocampus (vHPC) and medial prefrontal cortex (mPFC). While the damage is more extensive in hippocampus, the evidence of treatments, such as deep brain stimulation, suggests that functional changes in prefrontal cortex may be more critical. We hypothesized that antidepressant non-response may result from an insufficiency of transmission from vHPC to mPFC.

METHOD

Antidepressant non-responsive Wistar Kyoto (WKY) rats were subjected to chronic mild stress (CMS), then treated with chronic daily administration of the antidepressant drug venlafaxine (VEN) and/or repeated weekly optogenetic stimulation (OGS) of afferents to mPFC originating from vHPC or dorsal HPC (dHPC).

RESULTS

As in many previous studies, CMS decreased sucrose intake, open-arm entries on the elevated plus maze (EPM), and novel object recognition (NOR). Neither VEN nor vHPC-mPFC OGS alone was effective in reversing the effects of CMS, but the combination of chronic VEN and repeated OGS restored normal behaviour on all three measures. dHPC-mPFC OGS restored normal behaviour in the EPM and NOR test irrespective of concomitant VEN treatment, and had no effect on sucrose intake.

CONCLUSIONS

The synergism between VEN and vHPC-mPFC OGS supports the hypothesis that the antidepressant non-responsiveness of WKY rats results from a failure of antidepressant treatment fully to restore transmission in the vHPC-mPFC pathway.

The Neurocircuitry of Posttraumatic Stress Disorder and Major Depression: Insights Into Overlapping and Distinct Circuit Dysfunction-A Tribute to Ron Duman

The neurocircuitry that contributes to the pathophysiology of posttraumatic stress disorder and major depressive disorder, psychiatric conditions that exhibit a high degree of comorbidity, likely involves both overlapping and unique structural and functional changes within multiple limbic brain regions. In this review, we discuss neurobiological alterations that are associated with posttraumatic stress disorder and major depressive disorder and highlight both similarities and differences that may exist between these disorders to argue for the existence of a shared neurobiology. We highlight the key contributions based on preclinical studies, emerging from the late Professor Ronald Duman's research, that have shaped our understanding of the neurocircuitry that contributes to both the etiopathology and treatment of major depressive disorder and posttraumatic stress disorder.

Stress-Related Dysfunction of Adult Hippocampal Neurogenesis-An Attempt for Understanding Resilience?

Newborn neurons in the adult hippocampus are regulated by many intrinsic and extrinsic cues. It is well accepted that elevated glucocorticoid levels lead to downregulation of adult neurogenesis, which this review discusses as one reason why psychiatric diseases, such as major depression, develop after long-term stress exposure. In reverse, adult neurogenesis has been suggested to protect against stress-induced major depression, and hence, could serve as a resilience mechanism. In this review, we will summarize current knowledge about the functional relation of adult neurogenesis and stress in health and disease. A special focus will lie on the mechanisms underlying the cascades of events from prolonged high glucocorticoid concentrations to reduced numbers of newborn neurons. In addition to neurotransmitter and neurotrophic factor dysregulation, these mechanisms include immunomodulatory pathways, as well as microbiota changes influencing the gut-brain axis. Finally, we discuss recent findings delineating the role of adult neurogenesis in stress resilience.

TGF-β/Smad Signalling in Neurogenesis: Implications for Neuropsychiatric Diseases

TGF-β/Smad signalling has been the subject of extensive research due to its role in the cell cycle and carcinogenesis. Modifications to the TGF-β/Smad signalling pathway have been found to produce disparate effects on neurogenesis. We review the current research on canonical and non-canonical TGF-β/Smad signalling pathways and their functions in neurogenesis. We also examine the observed role of neurogenesis in neuropsychiatric disorders and the relationship between TGF-β/Smad signalling and neurogenesis in response to stressors. Overlapping mechanisms of cell proliferation, neurogenesis, and the development of mood disorders in response to stressors suggest that TGF-β/Smad signalling is an important regulator of stress response and is implicated in the behavioural outcomes of mood disorders.

Berberine and Ginsenoside Rb1 Ameliorate Depression-Like Behavior in Diabetic Rats

Rhizoma coptidis (Huang-lian) and Asian ginseng have been widely used in the treatment of diabetes and other concurrent diseases with apparent effects. This study investigated the effects of the active ingredients of R. coptidis and ginseng, berberine and ginsenoside Rb1, on depression-like behavior in a rat diabetes model. The animal model was established via a high-fat diet and intraperitoneal injection of streptozotocin, while the animal's depression-like behavior was induced via chronic unpredictable mild stress. These experimental rats were divided into four groups: control, depression-like behavior (DLB), metformin plus fluoxetine hydrochloride (M+FH), and berberine plus ginsenoside Rb1 (B+GRb1) groups. Glucose metabolism and insulin resistance were evaluated by oral glucose test and glucose clamp study. Depression-like behavior was evaluated via behavioral analyses, including forced swim, sucrose preference, elevated plus maze, and open-field tests. HE and Nissl staining, plasma cortisol expression of adrenocorticotropic hormone, and brain-derived neurotrophic factor (BDNF) levels were assayed to explore the mechanisms of action. Compared with the control, rats in the DLB group had a significant increase in the levels of blood glucose and depression-like behavior. The B+GRb1 group significantly improved glucose metabolism and insulin resistance, reduced depression-like behavior, downregulated levels of plasma cortisol and adrenocorticotropic hormone under stress, and upregulated BDNF protein expression compared to the DLB rats. HE and Nissl staining data revealed that B+GRb1 protected neurons from pathological and morphological changes. Thus, berberine and ginsenoside Rb1 not only improved glucose metabolism in diabetic rats but also ameliorated their depression-like behavior under chronic unpredictable stress. Mechanistically, studied data with plasma hormonal levels and brain neuronal pathological/morphological changes supported the observed effects. The combination of berberine and ginsenoside Rb1 may have a clinical value in the management of diabetic patients with depression.

Chronic stress induced perivascular adipose tissue impairment of aortic function and the therapeutic effect of exercise

NEW FINDINGS

What is the central question of this study? Thoracic perivascular adipose tissue (tPVAT) is known to, in part, regulate aortic function: what are the effects of unpredictable chronic mild stress (UCMS) on the tPVAT regulation of aortic function and what is the role of exercise training in alleviating the potential negative actions of UCMS on tPVAT? What is the main finding and its importance? UCMS causes tPVAT to disrupt endothelium-dependent dilatation, increases inflammatory cytokine production and diminishes tPVAT-adiponectin. Exercise training proved efficacious in preventing tPVAT-mediated disruption of aortic function. The data support a tPVAT mechanism through which chronic stress negatively impacts vascular health, which adds to our knowledge of how psychological disorders might increase the risk of cardiovascular disease.

ABSTRACT

Chronic stress is a major risk for cardiovascular disease. Perivascular adipose tissue (PVAT) has been shown to regulate vascular function; however, the impact of chronic stress and the comorbidity of metabolic syndrome (MetS) on thoracic (t)PVAT is unknown. Additionally, aerobic exercise training (AET) is known to combat the pathology of MetS and chronic stress, but the role of tPVAT in these actions is also unknown. Therefore, the purpose of this study was to examine the effects of unpredictable chronic mild stress (UCMS) on the tPVAT regulation of aortic function and the preventative effect of AET. Lean (LZR) and obese (OZR) Zucker rats (16-17 weeks old) were exposed to 8 weeks of UCMS with and without treadmill exercise (AET). In LZR, UCMS impaired aortic endothelium-dependent dilatation (EDD) (assessed ex vivo by wire myography) and aortic stiffness (assessed by elastic modulus) with no change in OZR subject to UCMS. However, both LZR and OZR UCMS tPVAT impaired EDD compared to respective controls. LZR and OZR subject to UCMS had higher oxidative stress production, diminished adiponectin and impaired aortic nitric oxide levels. Divergently, UCMS induced greater inflammatory cytokine production in LZR UCMS tPVAT, but not in OZR UCMS tPVAT. AET prevented the tPVAT impairment of aortic relaxation with UCMS in LZR and OZR. Additionally, AET reduced aortic stiffness in both LZR and OZR. These beneficial effects on tPVAT regulation of the aorta are likely due to AET preservation of adiponectin, reduced oxidative stress and inflammation, and enhanced nitric oxide. UCMS impaired tPVAT-regulated aortic function in LZR, and augmented MetS-induced EDD in OZR. Conversely, AET in combination with UCMS largely preserved aortic function and the tPVAT environment, in both groups.

Understanding the effects of chronic benzodiazepine use in depression: a focus on neuropharmacology

Benzodiazepines are frequently prescribed on an ongoing basis to individuals with depression, mainly to alleviate anxiety or insomnia, despite current guideline recommendations that continuous use should not exceed 4 weeks. Currently, there are no efficacy trials published beyond 8 weeks. Several antidepressant trials demonstrate that the concomitant use of a benzodiazepine is associated with poorer depressive outcomes and functional status; however, it is unclear why this is the case. Patients with depression receiving a benzodiazepine may reflect a more ill or high anxiety group, although even within anxiety disorders, the use of a benzodiazepine is associated with poorer outcomes. The neuroadaptive consequences of long-term benzodiazepine use may be a factor underlying these findings. Chronic benzodiazepine use results in decreased gamma-aminobutyric acid and monoaminergic function, as well as interference with neurogenesis, which are all purported to play a role in antidepressant efficacy. This review will discuss the oppositional neuropharmacological interactions between chronic benzodiazepine use and antidepressant mechanism of action, which could result in reduced antidepressant efficacy and function in depression.

Stress-Induced Morphological, Cellular and Molecular Changes in the Brain-Lessons Learned from the Chronic Mild Stress Model of Depression

Major depressive disorder (MDD) is a severe illness imposing an increasing social and economic burden worldwide. Numerous rodent models have been developed to investigate the pathophysiology of MDD. One of the best characterized and most widely used models is the chronic mild stress (CMS) model which was developed more than 30 years ago by Paul Willner. More than 2000 published studies used this model, mainly to assess novel compounds with potential antidepressant efficacy. Most of these studies examined the behavioral consequences of stress and concomitant drug intervention. Much fewer studies focused on the CMS-induced neurobiological changes. However, the stress-induced cellular and molecular changes are important as they may serve as potential translational biomarkers and increase our understanding of the pathophysiology of MDD. Here, we summarize current knowledge on the structural and molecular alterations in the brain that have been described using the CMS model. We discuss the latest neuroimaging and postmortem histopathological data as well as molecular changes including recent findings on microRNA levels. Different chronic stress paradigms occasionally deliver dissimilar findings, but the available experimental data provide convincing evidence that the CMS model has a high translational value. Future studies examining the neurobiological changes in the CMS model in combination with clinically effective antidepressant drug intervention will likely deliver further valuable information on the pathophysiology of MDD.

The Impact of Stress and Major Depressive Disorder on Hippocampal and Medial Prefrontal Cortex Morphology

Volumetric reductions in the hippocampus and medial prefrontal cortex (mPFC) are among the most well-documented neural abnormalities in major depressive disorder (MDD). Hippocampal and mPFC structural reductions have been specifically tied to MDD illness progression markers, including greater number of major depressive episodes (MDEs), longer illness duration, and nonremission/treatment resistance. Chronic stress plays a critical role in the development of hippocampal and mPFC deficits, with some studies suggesting that these deficits occur irrespective of MDE occurrence. However, preclinical and human research also points to other stress-mediated neurotoxic processes, including enhanced inflammation and neurotransmitter disturbances, which may require the presence of an MDE and contribute to further brain structural decline as the illness advances. Specifically, hypothalamic-pituitary-adrenal axis dysfunction, enhanced inflammation and oxidative stress, and neurotransmitter abnormalities (e.g., serotonin, glutamate, gamma-aminobutyric acid) likely interact to facilitate illness progression in MDD. Congruent with stress sensitization models of MDD, with each consecutive MDE it may take lower levels of stress to trigger these neurotoxic pathways, leading to more pronounced brain volumetric reductions. Given that stress and MDD have overlapping and distinct influences on neurobiological pathways implicated in hippocampal and mPFC structural decline, further work is needed to clarify which precise mechanisms ultimately contribute to MDD development and maintenance.

Investigating dopamine and glucocorticoid systems as underlying mechanisms of anhedonia

RATIONALE

Anhedonia, a deficit in reward processing, is an endophenotype of several neuropsychiatric conditions. Despite its prevalence and debilitating effects, treatments for anhedonia are lacking, primarily because its underlying mechanisms are poorly understood. Dopamine (DA) has been implicated in anhedonia through its role in reward-related learning; glucocorticoid systems may also be involved in that anhedonia is often preceded by chronic stress.

OBJECTIVE

This study investigated DA and glucocorticoid systems in anhedonia using a rat version of the probabilistic reward task (PRT).

METHODS

Adult male Wistar rats were trained on the PRT and then tested following: (1) activation or inhibition of DA activity induced by amphetamine (AMPH) or pramipexole (PRAMI) injections, (2) chronic mild stress (CMS), or (3) glucocorticoid system activation (dexamethasone (DEX)) or inhibition (mifepristone (MIFE)).

RESULTS

AMPH increased and PRAMI decreased response bias, pointing to enhanced and diminished reward responsiveness with DA agonism and antagonism, respectively. CMS reduced response bias but only in a subpopulation of rats. DEX also decreased response bias, suggesting that glucocorticoid processes contribute to anhedonia, although glucocorticoid inhibition (MIFE) had no effect. None of the manipulations altered the ability to detect and respond to reward-paired stimuli.

CONCLUSIONS

These results confirm a role of DA in anhedonia and elucidate the contribution of the glucocorticoid system to this effect. In addition, chronic stress may interfere with normal DA functioning, leading to impaired reward-related learning in some animals. These findings may direct future treatment of anhedonia by targeting DA and glucocorticoid systems, as well as a possible interaction between the two.

Effect of chronic unpredictable stress on mice with developmental under-expression of the Ahi1 gene: behavioral manifestations and neurobiological correlates

The Abelson helper integration site 1 (Ahi1) gene plays a pivotal role in brain development and is associated with genetic susceptibility to schizophrenia, and other neuropsychiatric disorders. Translational research in genetically modified mice may reveal the neurobiological mechanisms of such associations. Previous studies of mice heterozygous for Ahi1 knockout (Ahi1+/-) revealed an attenuated anxiety response on various relevant paradigms, in the context of a normal glucocorticoid response to caffeine and pentylenetetrazole. Resting-state fMRI showed decreased amygdalar connectivity with various limbic brain regions and altered network topology. However, it was not clear from previous studies whether stress-hyporesponsiveness reflected resilience or, conversely, a cognitive-emotional deficit. The present studies were designed to investigate the response of Ahi1+/- mice to chronic unpredictable stress (CUS) applied over 9 weeks. Wild type (Ahi1+/+) mice were significantly affected by CUS, manifesting decreased sucrose preference (p < 0.05); reduced anxiety on the elevated plus maze and light dark box and decreased thigmotaxis in the open field (p < 0.01 0.05); decreased hyperthermic response to acute stress (p < 0.05); attenuated contextual fear conditioning (p < 0.01) and increased neurogenesis (p < 0.05). In contrast, Ahi1+/- mice were indifferent to the effects of CUS assessed with the same parameters. Our findings suggest that Ahi1 under-expression during neurodevelopment, as manifested by Ahi1+/- mice, renders these mice stress hyporesponsive. Ahi1 deficiency during development may attenuate the perception and/or integration of environmental stressors as a result of impaired corticolimbic connectivity or aberrant functional wiring. These neural mechanisms may provide initial clues as to the role Ahi1 in schizophrenia and other neuropsychiatric disorders.

Radiation-induced overexpression of transthyretin inhibits retinol-mediated hippocampal neurogenesis

Cranial irradiation is the main therapeutic strategy for treating primary and metastatic brain tumors. However, radiation is well-known to induce several unexpected side effects including emotional disorders. Although radiation-induced depression may cause decreased quality of life after radiotherapy, investigations of its molecular mechanism and therapeutic strategies are still insufficient. In this study, we found that behavioral symptoms of depression on mice models with the decrease of BrdU/NeuN- and Dcx-positive populations and MAP-2 expression in hippocampus were induced by cranial irradiation, and transthyretin (TTR) was highly expressed in hippocampus after irradiation. It was shown that overexpression of TTR resulted in the inhibition of retinol-mediated neuritogenesis. PAK1 phosphorylation and MAP-2 expression were significantly reduced by TTR overexpression following irradiation. Moreover, we observed that treatment of allantoin and neferine, the active components of Nelumbo nucifera, interrupted irradiation-induced TTR overexpression, consequently leading to the increase of PAK1 phosphorylation, neurite extension, BrdU/NeuN- and Dcx-positive populations, and MAP-2 expression. Behavioral symptoms of depression following cranial irradiation were also relieved by treatment of allantoin and neferine. These findings demonstrate that TTR plays a critical role in neurogenesis after irradiation, and allantoin and neferine could be potential drug candidates for recovering the effects of radiation on neurogenesis and depression.

Differential microstructural alterations in rat cerebral cortex in a model of chronic mild stress depression

Chronic mild stress leads to depression in many cases and is linked to several debilitating diseases including mental disorders. Recently, neuronal tracing techniques, stereology, and immunohistochemistry have revealed persistent and significant microstructural alterations in the hippocampus, hypothalamus, prefrontal cortex, and amygdala, which form an interconnected system known as the stress circuit. Most studies have focused only on this circuit, however, some studies indicate that manipulation of sensory and motor systems may impact genesis and therapy of mood disorders and therefore these areas should not be neglected in the study of brain microstructure alterations in response to stress and depression. For this reason, we explore the microstructural alterations in different cortical regions in a chronic mild stress model of depression. The study employs ex-vivo diffusion MRI (d-MRI) to assess cortical microstructure in stressed (anhedonic and resilient) and control animals. MRI is followed by immunohistochemistry to substantiate the d-MRI findings. We find significantly lower extracellular diffusivity in auditory cortex (AC) of stress groups and a significantly higher fractional anisotropy in the resilient group. Neurite density was not found to be significantly higher in any cortical ROIs in the stress group compared to control, although axonal density is higher in the stress groups. We also report significant thinning of motor cortex (MC) in both stress groups. This is in agreement with recent clinical and preclinical studies on depression and similar disorders where significant microstructural and metabolic alterations were found in AC and MC. Our findings provide further evidence that the AC and MC are sensitive towards stress exposure and may extend our understanding of the microstructural effects of stress beyond the stress circuit of the brain. Progress in this field may provide new avenues of research to help in diagnosis and treatment intervention for depression and related disorders.

Understanding the pathophysiology of depression: From monoamines to the neurogenesis hypothesis model - are we there yet?

A number of factors (biogenic amine deficiency, genetic, environmental, immunologic, endocrine factors and neurogenesis) have been identified as mechanisms which provide unitary explanations for the pathophysiology of depression. Rather than a unitary construct, the combination and linkage of these factors have been implicated in the pathogenesis of depression. That is, environmental stressors and heritable genetic factors acting through immunologic and endocrine responses initiate structural and functional changes in many brain regions, resulting in dysfunctional neurogenesis and neurotransmission which then manifest as a constellation of symptoms which present as depression.

Stress and Loss of Adult Neurogenesis Differentially Reduce Hippocampal Volume

BACKGROUND

Hippocampal volume loss is a hallmark of clinical depression. Chronic stress produces volume loss in the hippocampus in humans and atrophy of CA3 pyramidal cells and suppression of adult neurogenesis in rodents.

METHODS

To investigate the relationship between decreased adult neurogenesis and stress-induced changes in hippocampal structure and volume, we compared the effects of chronic unpredictable restraint stress and inhibition of neurogenesis in a rat pharmacogenetic model.

RESULTS

Chronic unpredictable restraint stress over 4 weeks decreased total hippocampal volume, reflecting loss of volume in all hippocampal subfields and in both dorsal and ventral hippocampus. In contrast, complete inhibition of adult neurogenesis for 4 weeks led to volume reduction only in the dentate gyrus. With prolonged inhibition of neurogenesis for 8 or 16 weeks, volume loss spread to the CA3 region, but not CA1. Combining stress and inhibition of adult neurogenesis did not have additive effects on the magnitude of volume loss but did produce a volume reduction throughout the hippocampus. One month of chronic unpredictable restraint stress and inhibition of adult neurogenesis led to atrophy of pyramidal cell apical dendrites in dorsal CA3 and to neuronal reorganization in ventral CA3. Stress also significantly affected granule cell dendrites.

CONCLUSIONS

The findings suggest that adult neurogenesis is required to maintain hippocampal volume but is not responsible for stress-induced volume loss.

The chronic mild stress (CMS) model of depression: History, evaluation and usage

Now 30 years old, the chronic mild stress (CMS) model of depression has been used in >1300 published studies, with a year-on-year increase rising to >200 papers in 2015. Data from a survey of users show that while a variety of names are in use (chronic mild/unpredictable/varied stress), these describe essentially the same procedure. This paper provides an update on the validity and reliability of the CMS model, and reviews recent data on the neurobiological basis of CMS effects and the mechanisms of antidepressant action: the volume of this research may be unique in providing a comprehensive account of antidepressant action within a single model. Also discussed is the use of CMS in drug discovery, with particular reference to hippocampal and extra-hippocampal targets. The high translational potential of the CMS model means that the neurobiological mechanisms described may be of particular relevance to human depression and mechanisms of clinical antidepressant action.

Biophysical modeling of high field diffusion MRI demonstrates micro-structural aberration in chronic mild stress rat brain

Depression is one of the leading causes of disability worldwide. Immense heterogeneity in symptoms of depression causes difficulty in diagnosis, and to date, there are no established biomarkers or imaging methods to examine depression. Unpredictable chronic mild stress (CMS) induced anhedonia is considered to be a realistic model of depression in studies of animal subjects. Stereological and neuronal tracing techniques have demonstrated persistent remodeling of microstructure in hippocampus, prefrontal cortex and amygdala of CMS brains. Recent developments in diffusion MRI (d-MRI) analyses, such as neurite density and diffusion kurtosis imaging (DKI), are able to capture microstructural changes and are considered to be robust tools in preclinical and clinical imaging. The present study utilized d-MRI analyzed with a neurite density model and the DKI framework to investigate microstructure in the hippocampus, prefrontal cortex, caudate putamen and amygdala regions of CMS rat brains by comparison to brains from normal controls. To validate findings of CMS induced microstructural alteration, histology was performed to determine neurite, nuclear and astrocyte density. d-MRI based neurite density and tensor-based mean kurtosis (MKT) were significantly higher, while mean diffusivity (MD), extracellular diffusivity (Deff) and intra-neurite diffusivity(D_L) were significantly lower in the amygdala of CMS rat brains. Deff was also significantly lower in the hippocampus and caudate putamen in stressed groups. Histological neurite density corroborated the d-MRI findings in the amygdala and reductions in nuclear and astrocyte density further buttressed the d-MRI results. The present study demonstrated that the d-MRI based neurite density and MKT can reveal specific microstructural changes in CMS rat brains and these parameters might have value in clinical diagnosis of depression and for evaluation of treatment efficacy.

Disturbed diurnal rhythm of three classical phase markers in the chronic mild stress rat model of depression

Disturbances of circadian rhythms have been suggested to be a causal factor in the development of major depressive disorder. However, the mechanisms underlying the association between circadian rhythm abnormalities and mood disorders are still unknown. In the current study the association between diurnal pattern of key phase markers (melatonin, corticosterone, and core body temperature) and anhedonic-like behavior was investigated using the highly validated rat chronic mild stress (CMS) model of depression. Phase marker measurements were done after 3.5 weeks of CMS in 48 control rats and 48 anhedonic-like rats at 6 time points within 24h. The results showed that anhedonic-like behavior associates with changes in all three phase markers: an increased dark phase melatonin secretion, an additional peak in corticosterone level in the beginning of the light phase, and hypothermia in the dark phase. The result adds to the validity of the CMS model in general and in particular to be adequate as a model for studying the chronobiology of depressive disorder.

Chronic selective serotonin reuptake inhibition modulates endothelial dysfunction and oxidative state in rat chronic mild stress model of depression

Major depression is known to be associated with cardiovascular abnormalities, and oxidative stress has been suggested to play a role. We tested the hypothesis that antidepressant treatment reduces oxidative stress and endothelial dysfunctions in the chronic mild stress (CMS) model of depression in rats. Rats with >30% reduction in sucrose intake after 4 wk of CMS were defined in the study as CMS-susceptible and compared with unstressed controls. Sixteen CMS-susceptible and eight unstressed rats were treated during weeks 5 to 8 of the CMS protocol with escitalopram. Escitalopram-treated rats with >20% recovery in the sucrose consumption during the last 2 wk of treatment were defined as escitalopram responders. Rats that did not reach these criteria were defined as escitalopram nonresponders. In the open field test, escitalopram responders demonstrated anxiolytic effect of treatment. In mesenteric small arteries, escitalopram affected neither NO nor cyclooxygenase-1 (COX-1)-mediated vasodilation. Escitalopram potentiated endothelium-dependent hyperpolarization-like response, which was suppressed in the vehicle-treated CMS-susceptible rats and reduced COX-2-dependent relaxation, which was elevated in the vehicle-treated CMS-susceptible rats. Escitalopram did not affect blood pressure and heart rate, which were elevated in the vehicle-treated CMS-susceptible rats. Oxidative stress markers were changed in association with CMS in liver, heart, and brain. Escitalopram normalized oxidative stress markers in the majority of tissues. This study demonstrates that the antidepressant effect of escitalopram is associated with partial improvement of endothelial function in small arteries affecting COX-2 and endothelium-dependent hyperpolarization-like pathways.

Conditional Inhibition of Adult Neurogenesis by Inducible and Targeted Deletion of ERK5 MAP Kinase Is Not Associated with Anxiety/Depression-Like Behaviors

Although there is evidence that adult neurogenesis contributes to the therapeutic efficacy of chronic antidepressant treatment for anxiety and depression disorders, the role of adult neurogenesis in the onset of depression-related symptoms is still open to question. To address this issue, we utilized a transgenic mouse strain in which adult neurogenesis was specifically and conditionally impaired by Nestin-CreER-driven, inducible knockout (icKO) of erk5 MAP kinase in Nestin-expressing neural progenitors of the adult mouse brain upon tamoxifen administration. Here, we report that inhibition of adult neurogenesis by this mechanism is not associated with an increase of the baseline anxiety or depression in non-stressed animals, nor does it increase the animal's susceptibility to depression after chronic unpredictable stress treatment. Our findings indicate that impaired adult neurogenesis does not lead to anxiety or depression.

Postnatal loss of hap1 reduces hippocampal neurogenesis and causes adult depressive-like behavior in mice

Depression is a serious mental disorder that affects a person’s mood, thoughts, behavior, physical health, and life in general. Despite our continuous efforts to understand the disease, the etiology of depressive behavior remains perplexing. Recently, aberrant early life or postnatal neurogenesis has been linked to adult depressive behavior; however, genetic evidence for this is still lacking. Here we genetically depleted the expression of huntingtin-associated protein 1 (Hap1) in mice at various ages or in selective brain regions. Depletion of Hap1 in the early postnatal period, but not later life, led to a depressive-like phenotype when the mice reached adulthood. Deletion of Hap1 in adult mice rendered the mice more susceptible to stress-induced depressive-like behavior. Furthermore, early Hap1 depletion impaired postnatal neurogenesis in the dentate gyrus (DG) of the hippocampus and reduced the level of c-kit, a protein expressed in neuroproliferative zones of the rodent brain and that is stabilized by Hap1. Importantly, stereotaxically injected adeno-associated virus (AAV) that directs the expression of c-kit in the hippocampus promoted postnatal hippocampal neurogenesis and ameliorated the depressive-like phenotype in conditional Hap1 KO mice, indicating a link between postnatal-born hippocampal neurons and adult depression. Our results demonstrate critical roles for Hap1 and c-kit in postnatal neurogenesis and adult depressive behavior, and also suggest that genetic variations affecting postnatal neurogenesis may lead to adult depression.

Although the majority of the neurons in the brain are generated during embryonic stage, new neurons are continuously being produced postnatally, and at a much lower rate in adulthood. As postnatal neurogenesis is a key component of the brain maturation process that creates dynamic ‘wirings’ in the brain necessary for an individual to grow, learn, and cope with the external world, attenuated postnatal neurogenesis may affect an individual’s mental stability, rendering a higher susceptibility to depression later in life. In the current study, we genetically ablated the expression of huntingtin-associated protein 1 (Hap1) in mice at various ages or in selective brain regions, and found that early loss of Hap1 significantly reduces postnatal hippocampal neurogenesis, and leads to adult depressive-like behavior. We also found c-kit as an effector to mediate the neurogenesis defect and adult depressive-like phenotype in mice lacking Hap1. The results provide the first genetic evidence to demonstrate the importance of postnatal neurogenesis in adult depression, and may offer new avenues in the prevention and treatment of depression. Our study also has potential implications to other adult-onset mental disorders.

To stress or not to stress: a question of models

Stress research is a rapidly evolving field that encompasses numerous disciplines ranging from neuroscience to metabolism. With many new researchers migrating into the field, navigating the hows and whys of specific research questions can sometimes be enigmatic given the availability of so many models in the stress field. Additionally, as with every field, there are many seemingly minor experimental details that can have dramatic influences on data interpretation, although many of these are unknown to those not familiar with the field. The aim of this overview is to provide some suggestions and points to guide researchers moving into the stress field and highlight relevant methodological points that they should consider when choosing a model for stress and deciding how to structure a study. We briefly provide a primer on the basics of endpoint measurements in the stress field, factors to consider when choosing a model for acute stress, the difference between repeated and chronic stress, and importantly, influencing variables that modulate endpoints of analysis in stress work.

Chronic stress reduces the number of GABAergic interneurons in the adult rat hippocampus, dorsal-ventral and region-specific differences

Major depressive disorder is a common and complex mental disorder with unknown etiology. GABAergic dysfunction is likely to contribute to the pathophysiology since disrupted GABAergic systems are well documented in depressed patients. Here we studied structural changes in the hippocampal GABAergic network using the chronic mild stress (CMS) model, as one of the best validated animal models for depression. Rats were subjected to 9 weeks of daily stress and behaviorally characterized using the sucrose consumption test into anhedonic and resilient animals based on their response to stress. Different subtypes of GABAergic interneurons were visualized by immunohistochemistry using antibodies for parvalbumin (PV), calretinin (CR), calbindin (CB), cholecystokinin (CCK), somatostatin (SOM), and neuropeptide Y (NPY). We used an unbiased quantification method to systematically count labeled cells in different subareas of the dorsal and ventral hippocampus. Chronic stress reduced the number of specific interneurons in distinct hippocampal subregions significantly. PV+ and CR+ neurons were reduced in all dorsal subareas, whereas in the ventral part only the CA1 was affected. Stress had the most pronounced effect on the NPY+ and SOM+ cells and reduced their number in almost all dorsal and ventral subareas. Stress had no effect on the CCK+ and CB+ interneurons. In most cases the effect of stress was irrespective to the behavioral phenotype. However, in a few specific areas the number of SOM+, NPY+, and CR+ neurons were significantly reduced in anhedonic animals compared to the resilient group. Overall, these data clearly demonstrate that chronic stress affects the structural integrity of specific GABAergic neuronal subpopulations and this should also affect the functioning of these hippocampal GABAergic networks.

Mechanisms for interferon-α-induced depression and neural stem cell dysfunction

New neurons generated by the neural stem cells (NSCs) in the adult hippocampus play an important role in emotional regulation and respond to the action of antidepressants. Depression is a common and serious side effect of interferon-α (IFN-α), which limits its use as an antiviral and antitumor drug. However, the mechanism(s) underlying IFN-induced depression are largely unknown. Using a comprehensive battery of behavioral tests, we found that mice subjected to IFN-α treatment exhibited a depression-like phenotype. IFN-α directly suppressed NSC proliferation, resulting in the reduced generation of new neurons. Brain-specific mouse knockout of the IFN-α receptor prevented IFN-α-induced depressive behavioral phenotypes and the inhibition of neurogenesis, suggesting that IFN-α suppresses hippocampal neurogenesis and induces depression via its receptor in the brain. These findings provide insight for understanding the neuropathology underlying IFN-α-induced depression and for developing new strategies for the prevention and treatment of IFN-α-induced depressive effects.

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IFN-α-treated mice show a depression-like phenotype in a behavioral test battery

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IFN-α directly suppresses NSC proliferation in adult hippocampus

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IFN-α suppresses neurogenesis and induced depression via its receptor in the brain

Chronic mild stress for modeling anhedonia

Major depressive disorder is a complex disease implicating many brain circuitries. The clinical symptomatology is inconsistent and heterogenous and the pathogenesis is a complicated interplay of genetic and environmental factors. The episodic and recurrent nature of the disease, as well as the fact that several symptoms are only verbally expressed, make it challenging to establish valid and legitimate animal models of this disease. The purpose of this review is to provide some background knowledge and overview of valid rodent models of depression with an emphasis on our own experience with a chronic mild stress model in modeling of anhedonia and cognitive impairments associated with depression. In a final concluding remark, a 'dying-forward' hypothesis, for development of depression, is suggested on the basis of mainly our own data on a hippocampal pathology.

Novel insights into depression and antidepressants: a synergy between synaptogenesis and neurogenesis?

Major depressive disorder has been associated with manifold pathophysiological changes. These include metabolic abnormalities in discreet brain areas; modifications in the level of stress hormones, neurotransmitters, and neurotrophic factors; impaired spinogenesis and synaptogenesis in crucial brain areas, such as the prefrontal cortex and the hippocampus; and impaired neurogenesis in the hippocampus. Antidepressant therapy facilitates remission by reversing most of these disturbances, indicating that these dysfunctions may participate causally in depressive symptomatology. However, few attempts have been made to integrate these different pathophysiologies into one model. The present chapter endeavors (1) to review the extant literature in the field, with particular focus on the role of neurogenesis and synaptogenesis in depression; (2) and to suggest a possible interplay between these two processes, as well as, describe the ways by which improving both neurogenesis and synaptogenesis may enable effective recovery by acting on a larger neuronal network.

The neurobiology of depression and antidepressant action

We present a comprehensive overview of the neurobiology of unipolar major depression and antidepressant drug action, integrating data from affective neuroscience, neuro- and psychopharmacology, neuroendocrinology, neuroanatomy, and molecular biology. We suggest that the problem of depression comprises three sub-problems: first episodes in people with low vulnerability ('simple' depressions), which are strongly stress-dependent; an increase in vulnerability and autonomy from stress that develops over episodes of depression (kindling); and factors that confer vulnerability to a first episode (a depressive diathesis). We describe key processes in the onset of a 'simple' depression and show that kindling and depressive diatheses reproduce many of the neurobiological features of depression. We also review the neurobiological mechanisms of antidepressant drug action, and show that resistance to antidepressant treatment is associated with genetic and other factors that are largely similar to those implicated in vulnerability to depression. We discuss the implications of these conclusions for the understanding and treatment of depression, and make some strategic recommendations for future research.

Vesicular signalling and immune modulation as hedonic fingerprints: proteomic profiling in the chronic mild stress depression model

Extensive preclinical research has focused at unravelling the underlying molecular mechanisms leading to depression and recovery. In this study, we investigated the quantitative changes in protein abundance in the ventral hippocampal granular cell layer. We compared different phenotypes from the chronic mild stress (CMS) model of depression using chronic administration with two selective serotonin reuptake inhibitors (SSRIs), escitalopram and sertraline. We isolated granular cells using Laser-Capture Microdissection (LCM) and we identified their regulated proteins using two-dimensional (2D) differential gel electrophoresis (DIGE) and tandem mass spectrometry (MS/MS). The majority of the proteins we identified were enzymes involved in different metabolic activities. Additional proteins were functionally classified as vesicular proteins and immune system proteins. Rab GDP dissociation inhibitor alpha (GDIA) and syntaxin-binding protein 1 (STXB1) were potential markers for stress reactivity. Dynamin 1 (DYN1), glutathione S-transferase omega-1 (GSTO1) and peroxiredoxin (PRDX6) were associated with treatment response. In addition, an imbalance between different post-translationally modified versions of DYN1 and GSTO1 potentially accounted for SSRI treatment refraction. In the present study, we searched for new markers of stress reactivity and treatment response as well as any underlying molecular mechanisms correlating to the development of anhedonia and antidepressant therapy refraction. Our results pointed towards an essential role of post-translational modifications in both vesicular and immune protein systems.

Depression and hippocampal neurogenesis: a road to remission?

Adult-generated hippocampal neurons are required for mood control and antidepressant efficacy, raising hopes that someday we can harness the power of new neurons to treat mood disorders such as depression. However, conflicting findings from preclinical research--involving stress, depression, and neurogenesis--highlight the complexity of considering neurogenesis as a road to remission from depression. To reconcile differences in the literature, we introduce the "neurogenic interactome," a platform from which to consider the diverse and dynamic factors regulating neurogenesis. We propose consideration of the varying perspectives--system, region, and local regulation of neurogenesis--offered by the interactome and exchange of ideas between the fields of learning and memory and mood disorder research to clarify the role of neurogenesis in the etiology and treatment of depression.

Neurobiology of chronic mild stress: parallels to major depression

The chronic mild (or unpredictable/variable) stress (CMS) model was developed as an animal model of depression more than 20 years ago. The foundation of this model was that following long-term exposure to a series of mild, but unpredictable stressors, animals would develop a state of impaired reward salience that was akin to the anhedonia observed in major depressive disorder. In the time since its inception, this model has also been used for a variety of studies examining neurobiological variables that are associated with depression, despite the fact that this model has never been critically examined to validate that the neurobiological changes induced by CMS are parallel to those documented in depressive disorder. The aim of the current review is to summarize the current state of knowledge regarding the effects of chronic mild stress on neurobiological variables, such as neurochemistry, neurochemical receptor expression and functionality, neurotrophin expression and cellular plasticity. These findings are then compared to those of clinical research examining common variables in populations with depressive disorders to determine if the changes observed following chronic mild stress are in fact consistent with those observed in major depression. We conclude that the chronic mild stress paradigm: (1) evokes an array of neurobiological changes that mirror those seen in depressive disorders and (2) may be a suitable tool to investigate novel systems that could be disturbed in depression, and thus aid in the development of novel targets for the treatment of depression.

Synergistic antidepressant-like action of gaboxadol and escitalopram

According to current theories on the etiopathogenesis and pathophysiology of depression, both GABAergic and monoaminergic transmitter systems are perturbed. Consequently, the present study addressed the putative antidepressant action of the sedative-hypnotic GABAA receptor agonist, gaboxadol, separately and in combination with the selective serotonin reuptake inhibitor (SSRI) escitalopram. The rat chronic mild stress model was used to test the chronic antidepressant properties of gaboxadol in this depression model. Sucrose intake used as a read-out on anhedonic-like behavior indicated that the drug response rate for gaboxadol (5 mg/kg/day, i.p.) was similar to that measured for escitalopram (5 mg/kg/day, i.p.), however, the rate increased when the two drugs were co-administered, suggesting a synergistic action. Using in vivo electrophysiological recordings in dorsal raphe nucleus (DRN) of anesthetised rats, the present results showed that one week treatment with gaboxadol (5 mg/kg/day, i.p.) or with escitalopram (5 mg/kg/day, i.p.), followed by a 24 h washout period, did not affect DRN 5-HT neuronal firing per se, but in rats treated with both drugs for one week, the firing rate of DRN 5-HT neurons was significantly increased. Immunohistochemical estimations of cell proliferation in the hippocampal dentate gyrus did not reveal any effect of gaboxadol on chronic mild stressed rats, indicating that neurogenesis per se is not systematically associated with recovery from anhedonic-like behavior. Taken together, our data reveal for the first time an antidepressant action of gaboxadol and indicate a synergistic mechanism, regarding rapid onset of action and efficacy, when co-administered with escitalopram.

Increased cell proliferation in the rat anterior cingulate cortex following neonatal hypoxia: relevance to schizophrenia

As a consequence of obstetric complications, neonatal hypoxia has been discussed as an environmental factor in the pathophysiology of schizophrenia. However, the biological consequences of hypoxia are unclear. The neurodevelopmental hypothesis of schizophrenia suggests that the onset of abnormal brain development and neuropathology occurs perinatally, whereas symptoms of the disease appear in early adulthood. In our animal model of chronic neonatal hypoxia, we have detected behavioral alterations resembling those known from schizophrenia. Disturbances in cell proliferation possibly contribute to the pathophysiology of this disease. In the present study, we used postnatal rats to investigate cell proliferation in several brain areas following neonatal hypoxia. Rats were repeatedly exposed to hypoxia (89 % N₂, 11 % O₂) from postnatal day (PD) 4–8. We then evaluated cell proliferation on PD 13 and 39, respectively. These investigations were performed in the anterior cingulate cortex (ACC), caudate-putamen (CPU), dentate gyrus, and subventricular zone. Rats exposed to hypoxia exhibited increased cell proliferation in the ACC at PD 13, normalizing at PD 39. In other brain regions, no alterations have been detected. Additionally, hypoxia-treated rats showed decreased CPU volume at PD 13. The results of the present study on the one hand support the assumption of chronic hypoxia influencing transient cell proliferation in the ACC, and on the other hand reveal normalization during ageing.

Leptin restores adult hippocampal neurogenesis in a chronic unpredictable stress model of depression and reverses glucocorticoid-induced inhibition of GSK-3β/β-catenin signaling

Stress and glucocorticoid stress hormones inhibit neurogenesis, whereas antidepressants increase neurogenesis and block stress-induced decrease in neurogenesis. Our previous studies have shown that leptin, an adipocyte-derived hormone with antidepressant-like properties, promotes baseline neurogenesis in the adult hippocampus. This study aimed to determine whether leptin is able to restore suppression of neurogenesis in a rat chronic unpredictable stress (CUS) model of depression. Chronic treatment with leptin reversed the CUS-induced reduction of hippocampal neurogenesis and depression-like behaviors. Leptin treatment elicited a delayed long-lasting antidepressant-like effect in the forced swim behavioral despair test, and this effect was blocked by ablation of neurogenesis with X-irradiation. The functional isoform of the leptin receptor, LepRb, and the glucocorticoid receptor (GR) were colocalized in hippocampal neural stem/progenitor cells in vivo and in vitro. Leptin treatment reversed the GR agonist dexamethasone (DEX)-induced reduction of proliferation of cultured neural stem/progenitor cells from adult hippocampus. Further mechanistic analysis revealed that leptin and DEX converged on glycogen synthase kinase-3β (GSK-3β) and β-catenin. While DEX decreased Ser9 phosphorylation and increased Tyr216 phosphorylation of GSK-3β, leptin increased Ser9 phosphorylation and attenuated the effects of DEX at both Ser9 and Tyr216 phosphorylation sites of GSK-3β. Moreover, leptin increased total level and nuclear translocation of β-catenin, a primary substrate of GSK-3β and a key regulator in controlling hippocampal neural progenitor cell proliferation, and reversed the inhibitory effects of DEX on β-catenin. Taken together, our results suggest that adult neurogenesis is involved in the delayed long-lasting antidepressant-like behavioral effects of leptin, and leptin treatment counteracts chronic stress and glucocorticoid-induced suppression of hippocampal neurogenesis via activating the GSK-3β/β-catenin signaling pathway.

Chronic mild stress-induced depression-like symptoms in rats and abnormalities in catecholamine uptake in small arteries

OBJECTIVE

Major depression and cardiovascular diseases have a strong comorbidity; however, the reason for this is unknown. In the chronic mild stress (CMS) model of depression, only a fraction of rats develop a major feature of depression-anhedonia-like behavior, whereas other rats are stress resilient. Previous studies suggested that CMS rats also have increased total peripheral vascular resistance.

METHODS

On the basis of CMS-induced changes of sucrose intake, a reliable measure for anhedonia, rats were divided into "resilient" and "anhedonic" groups. An interaction between hedonic status and vascular function was studied after 4 and 8 weeks of CMS exposure in vitro in wire myograph on saphenous arteries and mesenteric small arteries (MSAs) from these rats.

RESULTS

When comparing the different experimental rat groups, arterial sensitivities to noradrenaline (NA) were similar under control conditions, but in the presence of the neuronal reuptake inhibitor cocaine, arteries from anhedonic rats were more sensitive to NA. No change in perivascular innervation was found, but elevated expression of neuronal NA transporter was detected. Inhibition of extraneuronal uptake with corticosterone (1 μM) suggests that this transport is diminished in MSAs after CMS. The corticosterone-sensitive transporter organic cation cotransporter 2 was shown to be reduced in MSAs after CMS. No CMS-induced changes in the corticosterone-sensitive transport were found in saphenous arteries.

CONCLUSIONS

Our results indicate that CMS-induced depression-like symptoms in rats are associated with changes in catecholamine uptake pathways in the vascular wall, which potentially modulates the effect of sympathetic innervation of resistance arteries.

Depression, antidepressants, and neurogenesis: a critical reappraisal

The neurogenesis hypothesis of depression posits (1) that neurogenesis in the subgranular zone of the dentate gyrus is regulated negatively by stressful experiences and positively by treatment with antidepressant drugs and (2) that alterations in the rate of neurogenesis play a fundamental role in the pathology and treatment of major depression. This hypothesis is supported by important experimental observations, but is challenged by equally compelling contradictory reports. This review summarizes the phenomenon of adult hippocampal neurogenesis, the initial and continued evidence leading to the development of the neurogenesis hypothesis of depression, and the recent studies that have disputed and/or qualified those findings, to conclude that it can be affected by stress and antidepressants under certain conditions, but that these effects do not appear in all cases of psychological stress, depression, and antidepressant treatment.

Chronic agomelatine treatment corrects behavioral, cellular, and biochemical abnormalities induced by prenatal stress in rats

RATIONALE AND OBJECTIVES

The rat model of prenatal restraint stress (PRS) replicates factors that are implicated in the etiology of anxious/depressive disorders. We used this model to test the therapeutic efficacy of agomelatine, a novel antidepressant that behaves as a mixed MT1/MT2 melatonin receptor agonist/5-HT(2c) serotonin receptor antagonist.

RESULTS

Adult PRS rats showed behavioral, cellular, and biochemical abnormalities that were consistent with an anxious/depressive phenotype. These included an increased immobility in the forced swim test, an anxiety-like behavior in the elevated plus maze, reduced hippocampal levels of phosphorylated cAMP-responsive element binding protein (p-CREB), reduced hippocampal levels of mGlu2/3 and mGlu5 metabotropic glutamate receptors, and reduced neurogenesis in the ventral hippocampus, the specific portion of the hippocampus that encodes memories related to stress and emotions. All of these changes were reversed by a 3- or 6-week treatment with agomelatine (40-50 mg/kg, i.p., once a day). Remarkably, agomelatine had no effect in age-matched control rats, thereby behaving as a "disease-dependent" drug.

CONCLUSIONS

These data indicate that agomelatine did not act on individual symptoms but corrected all aspects of the pathological epigenetic programming triggered by PRS. Our findings strongly support the antidepressant activity of agomelatine and suggest that the drug impacts mechanisms that lie at the core of anxious/depressive disorders.